Ever wonder how users truly feel about a product’s ease of use? The System Usability Scale (SUS) cuts through the noise with a simple, powerful tool that delivers reliable usability insights—fast.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a widely adopted, standardized questionnaire designed to assess the perceived usability of a system, product, or service. Developed in the late 1980s by John Brooke at Digital Equipment Corporation, SUS has since become a gold standard in usability evaluation across industries—from software and websites to medical devices and consumer electronics.
Origins and Development of SUS
The SUS was created during a time when usability testing was gaining traction but lacked a consistent, scalable method for measuring user experience. John Brooke introduced the 10-item questionnaire in 1986 as a way to quickly evaluate system usability without requiring extensive user testing or complex analytics.
Brooke’s original work was published in a technical report, not a peer-reviewed journal, which initially limited its visibility.Despite this, SUS gained popularity due to its simplicity, reliability, and flexibility across different domains.It was later validated through numerous academic studies, cementing its status in UX research..
“The beauty of SUS lies in its brevity and robustness.It doesn’t tell you why a system is hard to use, but it tells you definitively whether it is.” — Jakob Nielsen, Nielsen Norman Group
How SUS Differs From Other Usability Metrics
While many usability tools focus on task success rates, time-on-task, or error rates, SUS zeroes in on the user’s subjective perception of usability.This makes it unique and complementary to behavioral metrics..
- Subjective vs. Objective: SUS measures perceived usability, not observed performance.
- Speed and Scalability: It takes less than 10 minutes to complete, making it ideal for large-scale testing.
- Language and Culture Neutral: With proper translation, SUS performs well across diverse populations.
Unlike longer surveys like the Usability Metric for User Experience (UMUX) or the Post-Study System Usability Questionnaire (PSSUQ), SUS strikes a balance between depth and efficiency.
Why the System Usability Scale Matters in UX Design
In today’s competitive digital landscape, user experience is a key differentiator. A product can be feature-rich, but if users find it confusing or frustrating, adoption will suffer. The System Usability Scale provides a quantifiable way to measure how usable a product feels to real users.
Driving Data-Backed Design Decisions
Designers and product teams often rely on intuition or stakeholder opinions. SUS introduces objective data into the decision-making process.
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- Teams can compare SUS scores before and after a redesign to measure improvement.
- It helps prioritize usability issues when resources are limited.
- Stakeholders appreciate a single-number score that’s easy to understand and track over time.
For example, a mobile banking app scoring 65 on SUS might be considered below average. After a redesign focused on simplifying navigation, the score jumps to 82—providing clear evidence of improved usability.
Supporting Agile and Iterative Development
In agile environments, where rapid iterations are the norm, SUS fits seamlessly into sprints.
- It can be administered after each usability test or prototype review.
- Teams can track SUS trends across versions to ensure usability doesn’t degrade with new features.
- It’s lightweight enough to use with small sample sizes (as few as 5–8 users).
According to research by Sauro and Lewis (MeasuringU), even small samples can yield reliable SUS scores due to the scale’s high internal consistency.
How to Administer the System Usability Scale
One of the reasons for SUS’s enduring popularity is its ease of administration. Whether you’re conducting lab tests, remote sessions, or post-launch surveys, SUS can be integrated with minimal effort.
The 10-Item SUS Questionnaire
The SUS consists of 10 statements, each rated on a 5-point Likert scale from Strongly Disagree (1) to Strongly Agree (5). The odd-numbered items are positively worded, while even-numbered items are negatively worded (reverse-scored).
1.I think that I would like to use this system frequently.2.I found the system unnecessarily complex.3.I thought the system was easy to use.4.I think that I would need the support of a technical person to be able to use this system.5.I found the various functions in this system were well integrated.6.
.I thought there was too much inconsistency in this system.7.I would imagine that most people would learn to use this system very quickly.8.I found the system very awkward to use.9.I felt very confident using the system.10.I needed to learn a lot of things before I could get going with this system.After collecting responses, the scoring algorithm converts raw answers into a single SUS score between 0 and 100..
Scoring the System Usability Scale
Calculating the SUS score involves a specific formula that accounts for reverse-scoring and normalization.
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- For odd-numbered items: Subtract 1 from the user response (e.g., a rating of 3 becomes 2).
- For even-numbered items: Subtract the user response from 5 (e.g., a rating of 3 becomes 2).
- Sum all adjusted values and multiply by 2.5 to get the final SUS score (range: 0–100).
For example, if a user’s adjusted sum is 30, the SUS score is 30 × 2.5 = 75.
“The SUS scoring method may seem quirky, but it’s mathematically sound and produces a reliable, normalized score.” — Jeff Sauro, MeasuringU
Online calculators and templates (like those on MeasuringU’s SUS Calculator) automate this process, reducing errors.
Interpreting System Usability Scale Scores
A SUS score is more than just a number—it’s a benchmark for usability performance. But understanding what the score means in context is crucial.
The SUS Grade Scale and Benchmarks
While SUS scores range from 0 to 100, a score of 68 is considered the average based on decades of aggregated data.
- Below 50: Poor usability. Users are likely frustrated and may abandon the system.
- 50–67: Below average. Significant usability issues may exist.
- 68–75: Average. Acceptable but room for improvement.
- 76–85: Good. Users find the system easy to use.
- 86–100: Excellent. Top-tier usability, often seen in best-in-class products.
For instance, Google’s search interface typically scores above 90, reflecting its intuitive design.
Comparative Analysis Using SUS
SUS is most powerful when used comparatively—pitting one design against another.
- A/B testing two checkout flows? Measure SUS for each to see which feels more usable.
- Comparing competitor products? SUS provides a level playing field for subjective evaluation.
- Tracking product evolution? Plot SUS scores over time to visualize usability trends.
According to Sauro and Lewis (2009), a difference of 2.6 points in SUS scores is statistically significant with 95% confidence when comparing two groups of 15 users.
Strengths and Limitations of the System Usability Scale
No tool is perfect. While SUS is widely respected, it’s essential to understand both its strengths and its limitations to use it effectively.
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Advantages of Using SUS
The System Usability Scale offers several compelling benefits that explain its longevity.
- Reliability: High internal consistency (Cronbach’s alpha typically > 0.9).
- Validity: Strong correlation with other usability metrics and user satisfaction.
- Speed: Quick to administer and score.
- Flexibility: Applicable to software, hardware, websites, apps, and even non-digital systems.
- Cost-Effective: No licensing fees or specialized tools required.
Its simplicity allows even small startups to conduct professional-grade usability assessments.
Criticisms and Common Pitfalls
Despite its strengths, SUS is not without criticism.
- Lack of Diagnostic Detail: SUS tells you that a system is usable, but not why or where it fails.
- Subjectivity: Relies on self-reported data, which can be influenced by mood, expectations, or recent experiences.
- Translation Challenges: While SUS is used globally, poor translations can skew results.
- Context Blindness: Doesn’t account for user expertise—novices and experts rate systems similarly.
To mitigate these, experts recommend pairing SUS with qualitative methods like think-aloud protocols or interviews.
Best Practices for Implementing the System Usability Scale
Getting accurate and actionable results from SUS requires more than just handing out a questionnaire. Context, timing, and methodology matter.
When to Administer SUS
Timing affects the validity of SUS results. Administer it at the right moment for the most accurate feedback.
- After Task Completion: Best practice is to give the SUS immediately after users complete a set of representative tasks.
- Post-Interaction: For standalone products, administer it after a natural usage session.
- Avoid Delayed Surveys: Don’t send SUS days after use—recall bias diminishes accuracy.
For example, in a usability lab test, SUS is typically the final step after all tasks are completed.
Ensuring Valid and Reliable Results
To maximize the reliability of your SUS data, follow these evidence-based practices.
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- Use a Representative Sample: Include users who reflect your actual or target user base.
- Randomize Question Order (Cautiously): While possible, changing the order may affect scoring logic—stick to the standard format unless validated.
- Combine with Behavioral Metrics: Pair SUS with task success, time-on-task, and error rates for a holistic view.
- Report Confidence Intervals: Especially with small samples, include a margin of error (e.g., SUS = 75 ± 5).
Tools like MeasuringU’s SUS calculator can generate confidence intervals automatically.
Advanced Applications of the System Usability Scale
Beyond basic usability testing, SUS is being adapted for innovative uses in research and industry.
SUS in Healthcare and Medical Devices
In high-stakes environments like healthcare, usability can be a matter of life and death. Regulatory bodies like the FDA encourage the use of SUS in human factors validation.
- Used to evaluate medical device interfaces (e.g., infusion pumps, EHR systems).
- Helps meet IEC 62366 standards for usability engineering.
- Supports risk mitigation by identifying confusing interfaces before market release.
A study published in Applied Ergonomics found that SUS scores correlated strongly with use errors in simulated clinical settings.
Adapting SUS for Emerging Technologies
As technology evolves, so does the application of SUS. It’s now used to evaluate voice assistants, AR/VR systems, and AI-driven interfaces.
- Researchers modify SUS slightly (e.g., changing ‘system’ to ‘voice assistant’) while preserving the core structure.
- Used in academic studies on chatbot usability and autonomous vehicle interfaces.
- Some propose SUS variants like the Short SUS (6 items) for ultra-fast testing.
However, purists argue that any modification risks compromising the scale’s validity.
Alternatives and Complementary Tools to SUS
While SUS is powerful, it’s not the only usability metric available. Understanding alternatives helps you choose the right tool for the job.
UMUX and UMUX-Lite
The Usability Metric for User Experience (UMUX) is a 4-item scale based on ISO 9241-11. UMUX-Lite reduces this to 2 items for even faster assessment.
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- UMUX correlates highly with SUS (r > 0.9).
- More aligned with international usability standards.
- Less established than SUS but gaining traction.
For teams needing ISO compliance, UMUX may be preferable.
Nielsen’s Post-Study System Usability Questionnaire (PSSUQ)
Developed by Jakob Nielsen, PSSUQ is a 16-item questionnaire that dives deeper into satisfaction, interface quality, and information quality.
- More detailed than SUS but takes longer to complete.
- Ideal for in-depth post-test evaluations.
- Often used in academic and enterprise research.
While SUS gives a quick snapshot, PSSUQ provides a richer, multi-dimensional view.
Net Promoter Score (NPS) and SUS
NPS measures loyalty (“How likely are you to recommend this product?”), while SUS measures usability. Used together, they offer a broader picture of user experience.
- High SUS + High NPS = Strong user satisfaction and advocacy.
- High SUS + Low NPS = Usable but perhaps not valuable or differentiated.
- Low SUS + High NPS = Users love it despite usability flaws (e.g., early-stage startups).
Combining both metrics helps product teams balance usability with emotional engagement.
What is a good System Usability Scale score?
A score of 68 is considered average. Anything above 75 is good, and above 85 is excellent. However, what’s “good” depends on your industry and competitors. For example, consumer apps often aim for 80+, while enterprise software might be acceptable at 70+.
Can I modify the SUS questionnaire?
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While you can adapt the wording (e.g., replacing ‘system’ with ‘app’), significant changes to the items or scale can invalidate the scoring model. If you must modify it, conduct a validation study or use established variants like UMUX-Lite.
How many users do I need for a reliable SUS score?
As few as 5–8 users can provide a reliable estimate, especially for formative testing. For summative or comparative studies, 15–20 users per group are recommended to achieve statistical power.
Is the System Usability Scale free to use?
Yes, the SUS is in the public domain and free for both commercial and academic use. No permission is required, though proper citation of Brooke’s original work is encouraged.
Can SUS be used for non-digital products?
Absolutely. SUS has been successfully applied to physical products like medical devices, ATMs, and even household appliances. The key is ensuring users interpret ‘system’ appropriately in context.
The System Usability Scale remains one of the most trusted tools in the UX researcher’s toolkit. Its blend of simplicity, reliability, and versatility makes it indispensable for evaluating how users perceive a product’s ease of use. While it doesn’t replace deeper qualitative analysis, SUS provides a fast, standardized way to quantify usability—enabling comparisons, tracking progress, and driving design improvements. Whether you’re a startup founder, UX designer, or product manager, mastering the SUS can give you a competitive edge in building user-centered products.
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